improve error message for "expected ~[T] but found a ~-box pattern"

[pnkfelix]

Consider this code:

fn f(a: ~[int]) {
    match a {
        ~[b] => { let b:int = b; println!("a: {:?} b: {:d}", a, b); }
        _ => { fail!(); }
    }
}
fn main() {
    let a : ~[int] = ~[1i];
    f(a);
}

When I attempt to compile it (rustc 0.9-pre (97cd495 2013-10-02 01:16:31 -0700)), I get an error like this:

% rustc /tmp/m.rs
/tmp/m.rs:3:8: 3:12 error: mismatched types: expected `~[int]` but found a ~-box pattern
/tmp/m.rs:3         ~[b] => { let b:int = b; println!("a: {:?} b: {:d}", a, b); }
                    ^~~~
error: aborting due to previous error

I do not find the message above useful. My main reaction is: "The whole pattern ~[b] does look like a so-called ~-box pattern to me, which seems like it should fit the needs of a ~[int]."

After inspecting the source code for _match.rs, I think the issue is that I need to explicitly borrow a and then match against &[b], like so:

fn f(a: ~[int]) {
    match &a {
        &[b] => { let b:int = b; println!("a: {:?} b: {:d}", a, b); }
        _ => { fail!(); }
}

though I do not understand why the above compiles, since I would have thought that borrowing a ~[int] via &a would mean I should use match clauses of the form e.g. &~[b], not &[b].

In any case, we need a better error message here, preferably one that cross-references further discussion elsewhere, such as an appropriate (hypothetical) section of the rust.md manual. Note that the current rust.md documentation on match does not suffice to explain what to do here.

[huonw]

FWIW, all vector matches are unadorned, e.g. the following all print true in IRC-rusti.

match ~[1i] { [1i] => true, _ => false }
match @[1i] { [1i] => true, _ => false }
match &[1i] { [1i] => true, _ => false }

so matching with a pattern like ~[] is matching on something of type ~(<sigil>[T]).

(I do think that either we should have the matches have sigils (e.g. to match strings), or the error message should be more useful.)

[thestinger]

I would love it if we just dropped ~[T] and called it Vec<T>, but it would lose support for managed pointers without a nice garbage collector with a rooting API.

[chklauser]

Same with strings:

match os::args()[1] {
    ~"echo"   => expr,
    ...
}

results in "mismatched types: expected ~str but found a ~-box pattern" which I find very confusing. Especially, since removing the box ~ causes "mismatched types: expected ~str but found &'static str (expected ~str but found &'static str)"

Using rust pre-nightly from 92f6b92

Edit (Had copied, the wrong error message before, corrected)

[nrc]

That second error sounds wrong because the types in the brackets don't match the real types. But I think that is a different issue (not sure if there is an issue filed).

This issue will either go away or change (hopefully be easier to fix) when DST is finished (#12938)

[chklauser]

Ah yes, you're right. I copied the wrong error message while playing around. rustc complains that ~str and &'static str don't match.

I corrected my earlier comment.

[ghost]

Fixed in #18171.